﻿using System;
using System.Linq.Expressions;

namespace HelperTools.Generics
{
	/// <summary>
	/// General purpose Expression utilities
	/// </summary>
	public static class ExpressionUtil
	{
		/// <summary>
		/// Create a function delegate representing a unary operation
		/// </summary>
		/// <typeparam name="TArg1">The parameter type</typeparam>
		/// <typeparam name="TResult">The return type</typeparam>
		/// <param name="body">Body factory</param>
		/// <returns>Compiled function delegate</returns>
		public static Func<TArg1, TResult> CreateExpression<TArg1, TResult>(Func<Expression, UnaryExpression> body)
		{
			ParameterExpression inp = Expression.Parameter(typeof(TArg1), "inp");
			try
			{
				return Expression.Lambda<Func<TArg1, TResult>>(body(inp), inp).Compile();
			}
			catch (Exception ex)
			{
				string msg = ex.Message; // avoid capture of ex itself
				return delegate { throw new InvalidOperationException(msg); };
			}
		}

		/// <summary>
		/// Create a function delegate representing a binary operation
		/// </summary>
		/// <typeparam name="TArg1">The first parameter type</typeparam>
		/// <typeparam name="TArg2">The second parameter type</typeparam>
		/// <typeparam name="TResult">The return type</typeparam>
		/// <param name="body">Body factory</param>
		/// <returns>Compiled function delegate</returns>
		public static Func<TArg1, TArg2, TResult> CreateExpression<TArg1, TArg2, TResult>(Func<Expression, Expression, BinaryExpression> body)
		{
			return CreateExpression<TArg1, TArg2, TResult>(body, false);
		}

		/// <summary>
		/// Create a function delegate representing a binary operation
		/// </summary>
		/// <param name="castArgsToResultOnFailure">
		/// If no matching operation is possible, attempt to convert
		/// TArg1 and TArg2 to TResult for a match? For example, there is no
		/// "decimal operator /(decimal, int)", but by converting TArg2 (int) to
		/// TResult (decimal) a match is found.
		/// </param>
		/// <typeparam name="TArg1">The first parameter type</typeparam>
		/// <typeparam name="TArg2">The second parameter type</typeparam>
		/// <typeparam name="TResult">The return type</typeparam>
		/// <param name="body">Body factory</param>
		/// <returns>Compiled function delegate</returns>
		public static Func<TArg1, TArg2, TResult> CreateExpression<TArg1, TArg2, TResult>(
			 Func<Expression, Expression, BinaryExpression> body, bool castArgsToResultOnFailure)
		{
			ParameterExpression lhs = Expression.Parameter(typeof(TArg1), "lhs");
			ParameterExpression rhs = Expression.Parameter(typeof(TArg2), "rhs");
			try
			{
				try
				{
					return Expression.Lambda<Func<TArg1, TArg2, TResult>>(body(lhs, rhs), lhs, rhs).Compile();
				}
				catch (InvalidOperationException)
				{
					if (castArgsToResultOnFailure && !(         // if we show retry                                                        
							  typeof(TArg1) == typeof(TResult) &&  // and the args aren't
							  typeof(TArg2) == typeof(TResult)))
					{ // already "TValue, TValue, TValue"...
					  // convert both lhs and rhs to TResult (as appropriate)
						Expression castLhs = typeof(TArg1) == typeof(TResult) ? (Expression)lhs : (Expression)Expression.Convert(lhs, typeof(TResult)); 
						Expression castRhs = typeof(TArg2) == typeof(TResult) ? (Expression)rhs : (Expression)Expression.Convert(rhs, typeof(TResult));

						return Expression.Lambda<Func<TArg1, TArg2, TResult>>(
							 body(castLhs, castRhs), lhs, rhs).Compile();
					}
					throw;
				}
			}
			catch (Exception ex)
			{
				string msg = ex.Message; // avoid capture of ex itself
				return delegate { throw new InvalidOperationException(msg); };
			}
		}
	}
}
